PROJECT SUMMARY Interventions to increase physical activity (PA) are vitally needed in adolescent and young adult childhood cancer survivors (AYA survivors) who are largely inactive but are at lifelong elevated risk of cardiomyopathy, insulin resistance, and other cardiometabolic disorders. To reach AYA survivors, their desire for peer connections, almost ubiquitous use of smartphones and social media, and strong preferences to exercise in their homes and communities and as part of social activity must be leveraged. The intervention must also be accessible and affordable for patients without private health insurance as they are less likely to receive survivorship care and more likely to have greater morbidity. The transdisciplinary study team has completed two recent pilot studies among AYA survivors that provide substantial scientific premise for the proposed research by demonstrating that the intervention is feasible, has high rates of uptake and adherence, and appears promising for increasing PA. The proposed multi-site, randomized controlled trial (RCT) will enroll 384 AYA survivors currently ages 15.0- 20.9 years who are 3-36 months off therapy and not meeting PA guidelines. The 12-month trial will test the efficacy of a 6-month intensive multi-level PA intervention combining a wearable Fitbit PA tracker (intrapersonal level) with integration of activity data leading to individualized goal setting by text (intrapersonal level) and a private Instagram account serving as a virtual peer support group of survivors (interpersonal and community level) followed by a 6-month maintenance phase to improve PA in AYA survivors. Given the ubiquity of PA tracking devices and apps, the control group will receive the Fitbit only without integration of Fitbit data in other activities. A nationwide sample of patients drawn from rural, urban, community, and academic centers will be recruited using the Children?s Oncology Group consortium of >200 hospitals. This rigorously designed, multi-site RCT will test the efficacy of the mobile health (mHealth) behavioral intervention, compared to the control group, to achieve the following Specific Aims among AYA survivors over 12 months: Aim 1 - To increase PA; Aim 2 - To improve biomarkers predictive of cardiometabolic health; and Aim 3 - To improve health-related quality of life. Analyses of hypothesized mediation factors for Aims 1-3 and post-trial qualitative interviews among participants with and without private health insurance will provide explanatory knowledge to help refine intervention procedures and tailor the intervention for future RCTs among patient subgroups who will most benefit. If shown to be efficacious, this mHealth intervention to improve PA in AYA survivors is highly scalable because smartphones and social media are widespread, PA trackers are popular and affordable, goal setting can be automated, and social media activities do not require specialized staff.